The present invention relates to a fully safely operating fire resistant electric cable.
As is known, fire or flame resistant electric cables must provide, as they are subjected to a direct flame and a consequent temperature variation from 750° C. to 930° C., a safe current flow for a time period from 1 to 2 hours.
Such temperature and flame exposition time variations will exclusively depend on different international standards, providing correspondingly different test methods.
Fire or flame resistant cables must be designed to assure a proper operation of the apparatus they are connected to, for limiting deleterious consequences of fire, and allowing lighting bodies and, in general, elements designed for assuring a quick evacuation of the fire affected regions to continuously operate at least for a set time period.
To solve the above mentioned problem, prior anti-fire electric cables conventionally comprise, on each individual copper conductor or wire thereof, a mica processed strip, i.e. a glass strip on a side of which a mica layer of a thickness of several microns is glued.
Thus, as the cable components are burnt away by the flame, which occurs in about 10-15 minutes, said thin mica layer will allow the cable to further operate even if for a very short time.
Thus, it should be apparent that the mica layer is the most critical part of the cable: actually a defective of said mica layer would cause electrical discharges and shorts thereby preventing the electric cable from further operating.
Another problem is that, in installing said cable it will be subjected to mechanical stresses susceptible to damage the mica layer and the cable insulating characteristics against outer agents.
In this connection it should be moreover pointed out that it is very difficult to provide a high strength, since, according to regulating standards, an electric cable may be considered as properly usable if, in testing operations, only a cable of three is discarded.
Accordingly, prior fire resistant electric cables have a poor fire performance, both due to a difficulty of properly applying the mica strip, and since said mica strip is quickly damaged by mechanical stress, thereby as the cable is installed it is so damaged that its waterproof properties are quickly loosen thereby generating electric shorts.
A prior approach to solve the above problem provides to use MICC cables including mineral insulation means which, however, are very expensive and require very difficult to be met installation condition, thereby they are only used in a very limited number of applications.